Flow meters utilizing ultrasonic transducers can be used to sense fluid flow properties such as velocity, volumetric flow rate, etc. Cross correlation ultrasonic flow meters (CCUF), for example, can measure the time required for ultrasonic beams to transit across a flow path at two, axially displaced locations along a pipe. Within this measurement principle, variations in transit time are assumed to correlate with properties that convect with the flow, such as vortical structure, inhomogeneities in flow composition, and temperature variations to name a few.
CCUFs utilize high frequency acoustic signals, i.e. ultrasonics, to measure much lower frequency, time varying properties of structures in the flow. Like all other cross correlation based flow meters, the physical disturbances which cause the transit time variations should retain some level of coherence over the distance between the two sensors. CCUFs are typically much more robust to variations in fluid composition than the other ultrasonic-based flow measurement approaches such as transit time and Doppler based methods.
Transit time, defined as the time required for an ultrasonic beam to propagate a given distance, can be measured using a radially aligned ultrasonic transmitter and receiver. For a homogenous fluid with a no transverse velocity components flowing in an infinitely rigid tube, the transit time is given by the following relation:t=D/Amix where “t” is the transit time, D is the diameter of the pipe, and Amix is the speed of sound propagating through the fluid.
In such a flow, a variation in transit time is analogous to a variation in sound speed of the fluid. In real fluids however, there are many mechanisms, which could cause small variations in transit time which remain spatially coherent for several pipe diameters. For single phase flows, variations in the transverse velocity component will cause variations in transit time. Variations in the thermophysical properties of a fluid such as temperature or composition will also cause variations. Many of these effects convect with the flow. Thus, influence of transverse velocity of the fluid associated with coherent vortical structures on the transit time enables transit time based measurements to be suitable for cross correlation flow measurement for flows with uniform composition properties. The combination of sensitivity to velocity field perturbation and to composition changes make transit time measurement well suited for both single and multiphase applications.
Despite CCUFs functioning over a wide range of flow composition, standard transit time ultrasonic flow meters (TTUF) are more widely used. TTUFs tend to require relatively well behaved fluids (i.e. single phase fluids) and well-defined coupling between the transducer and the fluid itself TTUFs rely on transmitting and receiving ultrasonic signals that have some component of their propagation in line with the flow. While this requirement does not pose a significant issue for in-line, wetted transducer TTUFs, it does pose a challenge for clamp-on devices by introducing the ratio of sound speed in the pipe to the fluid as an important operating parameter. The influence of this parameter leads to reliability and accuracy problems with clamp-on TTUFs.
Signal-to-noise ratio (i.e., the ratio of a desired signal to a noise signal containing no useful information) is very often an issue with flow meters that utilize non-wetted ultrasonic sensors that send and receive signals through the walls of the vessel (e.g., pipe) in which the fluid flow is passing. In addition, differences in material properties between the pipe walls and the fluid flow traveling therein can create impedance mismatches that inhibit signal propagation. Attenuated signals undesirably decrease the signal-to-noise ratio, and likely also decrease the accuracy of information available from the signal. Consequently, it would be desirable to provide a method for improving the strength and quality of a signal produced and received by an ultrasonic sensor unit utilized within a flow meter, and an apparatus operable to do the same.